1. Field of the Invention
This invention relates to time-scale modification methods and apparatuses that perform time-scale modification on digital signals without changing original pitches in accordance with time-scale modification factors.
This application is based on Patent Application No. Hei 11-126343 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, engineers and scientists propose time-scale modification techniques to compress or expand digital audio signals with respect to time without changing original pitches. For example, those techniques are used for the so-called xe2x80x9cscale adjustmentxe2x80x9d, in which an overall recording time for recording digital audio signals is adjusted to a prescribed time, and xe2x80x9ctempo modificationxe2x80x9d used by Karaoke devices. A cut-and-splice method is conventionally known as one kind of the time-scale modification techniques. According to this method whose operations are shown in FIGS. 9A, 9B, original digital audio signals S having waveforms (or envelopes) are sequentially divided into and cut to wave segments having prescribed time lengths, so that the wave segments are spliced together. Herein, discontinuity is caused to occur at joints at which the wave segments are jointed together. To eliminate the discontinuity, cross-fade processes are effected on the joints between the wave segments so that the wave segments are being smoothly connected together. A time-scale modification factor R is expressed by an equation (1), as follows:                     R        =                  Ls                      Ls            +            Loff                                              (        1        )            
where Ls denotes a cutting length used for cutting original waves, and Loff denotes an offset length which lies between a back-end portion of a wave segment being cut and its next wave segment.
FIG. 9A shows an example of time-scale expansion, wherein the offset length Loff has a negative value, so that R greater than 1. FIG. 9B shows an example of time-scale compression, wherein the offset length Loff has a positive value, so that R less than 1. Therefore, when certain values are given as the time-scale modification factor R and cutting length Ls respectively, the offset length Loff is calculated directly from an equation (2), as follows:                     Loff        =                                            1              -              R                        R                    ·          Ls                                    (        2        )            
According to the conventional time-scale modification techniques, wave segments are spliced together at prescribed positions corresponding to the offset length Loff, which is determined and set in response to the time-scale modification factor, regardless of conditions of the waves. For this reason, although the cross-fade processes are effected on joints of the wave segments, phase deviations are caused to occur at the joints of the wave segments. This causes deterioration of sound quality in reproduction of sounds which are reproduced by way of time-scale modification.
It is an object of the invention to provide a time-scale modification method or apparatus which is capable of compressing or expanding digital signals in accordance with desired time-scale modification factors without causing deterioration in sound quality at joints of wave segments, which are cut from original waves of the digital signals and are spliced together.
According to a time-scale modification method or apparatus of this invention, wave segments each having a prescribed cutting length are sequentially cut from original digital signal waves stored in a waveform memory and are then spliced together by way of cross-fading, so it is possible to realize time-scale modification (i.e., compression or expansion with respect to time) in accordance with a designated time-scale modification factor. Herein, time-scale modification parameters such as a cross-fade duration, a search start time and a search end time are produced in response to the designated time-scale modification factor. In addition, a cutting start position is used for cutting a next wave segment following a present wave segment. The cutting start position is determined within a period of time between the search start time and search end time in such a way that it is placed to provide a best similarity between the wave segments having prescribed portions which are connected with each other by way of cross-fading. Specifically, a back-end portion of the present wave segment and a top portion of the next wave segment are smoothly connected together by way of the cross-fading, wherein they have the same cross-fade duration. The cross-fade duration is controlled to be longer as the time-scale modification factor becomes greater or smaller than xe2x80x9c1xe2x80x9d. The cross-fading is actualized by a window function having different cross-fade coefficients, which are varied over a lapse of time and by which data of the prescribed portions of the wave segments are multiplied and mixed together.
Thus, it is possible to provide smooth connections between the wave segments which are cut to provide the best similarity and are spliced together by way of the cross-fading, so it is possible to actualize advanced time-scale modification in which sound quality is not deteriorated so much at joints of the wave segments in reproduced sounds.